[I wrote the following after Tal's first response, before reading Raymond's second post in response to Tal.]

The October releases were deficient in only documenting Squeezer in the IDLE section of "What's New in Python X.Y" and a News entry (copied to Help => About IDLE => News).  I don't intend to repeat mistake like this.

As part of working through the backlog of IDLE doc issues, I have since added the following.
"""
Startup and code execution [existing section]
...
User output in Shell [new subsection]
...
Shell has a special facility for squeezing output lines down to a ‘Squeezed text’ label.  This is done automatically for output over N lines (N = 50 by default). N can be changed in the PyShell section of the General page of the Settings dialog.  Output with fewer lines can be squeezed by right clicking on the output.  This can be useful [for] lines long enough to slow down scrolling. 

Squeezed output is expanded in place by double-clicking the label. It can also be sent to the clipboard or a separate view window by right-clicking the label.   
"""

I just noticed the missing 'for'.  I am thinking of rewriting the sentence as "This can be useful when lines are so long that they make scrolling slower."

Adding to what Tal said: IDLE's calltips replace some uses of help() in standard interactive Python.  For the rest, I think being able to move hundreds of lines out of the REPL and into a separate persistent window, which that can be moved at least partly aside from Shell, improves IDLE's usability for teaching.  Such blobs of text make it hard to scroll back to see previous entries and responses.

Overall, partial squeezing does not seem like a great idea to me.  It only partially fixes the scrolling issue.  It would complicate both the implementation and use of squeezing, especially for viewing the whole text outside of Shell.  I looked at the first 100 lines of help(str) and 65 (after the first 15) list the generic dunder methods.  I think showing all either in Shell or a text view is better.